Tyres – not just black and round
Tyre manufacturers have a problem: their product isn’t sexy. While enthusiasts scour car magazines, ogling the latest spoiler, steering wheel or bucket seat, the dull black objects that connect the car to the road are barely mentioned. For the majority of motorists, tyres are an afterthought, a necessary evil that must be maintained and then replaced, often at considerable expense.
Such a view is understandable – tyres are not visually appealing – but it is also misguided. The tyres are the only contact your car has with the road - every braking, acceleration or steering force must be transmitted through a ‘contact patch’ no larger than a postcard. Your choice of tyre will therefore determine how your car performs and making the wrong choice could have disastrous results. The following information helps unravel the mysteries of tyre choice and offers some practical tips for maximising their performance.
THE BASICSModern tyres are not just giant lumps of rubber moulded into shape. Although some natural rubber is still used in their construction, they are now a complex blend of a huge range of materials including steel, rayon, nylon, polyester and carbon black. The exact composition of this compound will determine the tyre’s characteristics. That’s why you will often hear Formula 1 teams talking about ‘hard’ and ‘soft’ compounds.
Tread patterns also differ according to how the tyre is to be used. In a perfect, dry world, the tyre would be smooth – like a ‘slick’ tyre used in motor racing – because this maximises the size of the contact patch: the quantity of tyre touching the road. But such a tyre is useless in wet conditions and so would be unsuitable for road use. The tread pattern of modern road tyres are therefore designed for an optimal compromise of dry and wet weather grip, while also taking into account cornering forces, road noise and ‘rolling resistance’ which impacts on the car’s fuel consumption.
CUTTING THROUGH THE JARGONEvery tyre has a sidewall marking – such as 185/70 R14 80 T – which identifies its size and characteristics. These markings can be analysed as follows:
|185||The width of the tyre in millimetres.|
|70||The aspect ratio – this is the ratio of the tyre’s width to its height or ‘sidewall’. A ’70 series’ tyre is a tyre whose height is equal to 70% of its width. Low profile tyres have shallow sidewalls and correspondingly low aspect ratings.|
|R||This denotes the tyre’s construction type – in this case it’s a radial.|
|14||The diameter of the wheel rim in inches|
|80||Load index – this represents the maximum load capacity of a tyre when it’s driven at maximum speed. A tyre with an Li of 80 can bear a maximum load of 450kg.|
|R||Speed rating – all tyres carry a speed rating which indicates the maximum speed for which the tyre was designed. An R rated tyre should not be driven above 170km/h (106mph). A V rated tyre, by contrast, is capable of speeds of up to 149mph.|
Tyre age – the sidewall also carries a three digit age code indicating when it was made. For example, 124 means the tyre was manufactured in December 2004.
The vast majority of cars are fitted with standard road tyres that provide an all-round blend of qualities. Many boast a block-shaped tread pattern to combine good wet and dry weather grip with ample steering control. They will also be tuned for low noise and reduced rolling resistance (for improved fuel consumption).
Standard road tyres
High performance tyresCompared with more conventional road tyres, high performance tyres tend to be wider and have a lower profile (shallower sidewall). Many will also boast asymmetrical or directional tread patterns. Asymmetrical tyres have different tread patterns on either side of the tyre to optimise the opposing requirements of dry weather grip and water dispersal. Directional tyres are designed for high speed running and are characterised by lateral grooves on both sides of the tyre pointing in the same direction, which helps with water dispersal. Both asymmetrical and directional tyres must be fitted to the car in the correct direction.
Low profile tyres provide improved handling and grip, as well as increased traction and braking performance. But drivers must weigh these advantages against increased road noise, a firmer ride and an increased risk of aquaplaning (when the tread fills with water and the tyres lose their grip on the road).
Winter tyresAll-but essential in countries with cold climates, winter tyres are suitable for use on snow and ice covered roads. A block tread pattern with deep grooves provides excellent braking performance and traction in icy conditions. The tyre compound is also tuned to provide good friction characteristics and suppleness in cold temperatures.
Off-road tyresOff-road tyres feature a chunky tread design that provides grip and stability in the varied and difficult conditions experienced away from the terra firma. Most standard 4x4 vehicles, such as the Range Rover, are supplied with tyres designed for on- rather than off-road use. Serious off-roaders will need to replace or supplement these with more aggressive tyres designed specially for all-terrain use.
Experienced off-road drivers will also adjust the tyre pressures to cope with the differing surfaces. In mud, snow or sand, for example, it’s appropriate to lower the tyre pressures to provide additional traction.
Run-flat tyresRun-flat tyres have become increasingly popular in recent years. Specially constructed to maintain their shape in the event of a puncture, a run-flat tyre can be driven at a reduced speed (80km/h (50mph)) even if it’s been ruptured. This removes the need for a spare tyre and alleviates the risks associated with stopping to change a damaged tyre. Some manufacturers, such as Mini, now fit run-flat tyres as standard in conjunction with an internal tyre pressure monitoring gauge. They can also be retro-fitted.
High performance tyresCareful maintenance of your tyres is important to ensure that they continue to operate at their maximum efficiency. This is not only an important safety concern; it can also increase driving pleasure.
Air pressure – it’s important to check your tyre pressures every month and to compare them with the correct ratings as described in the vehicle handbook. Under-inflated tyres will wear more quickly and there is a greater risk of a puncture. Over-inflated tyres also puncture more easily and can create erratic handling characteristics.
Tread depth - the minimum legal tread depth varies from country to country – in the UK it’s 1.6mm – but tyre specialists suggest a depth of less than 3mm can prove problematic. The tyre tread depth must extend across three-quarters of the tyre width. Badly worn tyres offer less grip and are more susceptible to a puncture.
Wheel nuts – regularly check that the wheelnuts are properly tightened.
Cleaning – never use detergents or chemicals containing petroleum products for cleaning or polishing your tyres.
Damaged tyres – never drive with damaged tyres, which carry an increased risk of a puncture.